Re: struct ifnet and ifaddr handling [was: Re: Making global variables of if.c MPSAFE]

[Masao Uebayashi <uebayasi%gmail.com@localhost>]

On Mon, Nov 24, 2014 at 11:16 PM, Taylor R Campbell
<campbell+netbsd-tech-kern%mumble.net@localhost> wrote:
>    Date: Mon, 24 Nov 2014 15:26:21 +0900
>    From: Masao Uebayashi <uebayasi%gmail.com@localhost>
>
>    (I'm trying, but I can't follow up all mails soon, because I need more
>    than x2 energy & time to write English than you.)
>
> I understand!  I will keep this one brief.
>
>    On Mon, Nov 24, 2014 at 12:12 PM, Taylor R Campbell
>    <campbell+netbsd-tech-kern%mumble.net@localhost> wrote:
>    > Can you (or ozaki-r@, whose earlier patch I missed until now) explain
>    > specifically what this accomplishes?  I have two guesses about the
>    > primary goal of this change: either
>    >
>    > (a) to obviate the need to run if_watchdog/if_slowtimo callbacks
>    > inside IFNET_FOREACH, or
>
>    I was thinking of this first.
>
> OK!
>
>    > In case (a), what might an interface do in an if_watchdog/if_slowtimo
>    > callback that is safe in a callout but not safe inside a pserialized
>    > reader?  Is it simply that it's sort-of-kind-of OK for a callout to
>    > block a little, but absolutely not OK for a pserialized reader to
>    > block and thus switch?
>
>    I have believed that pserialize(9) reader-side is a critical section.
>    pserialize(9) relies on scheduler to notify that readers have passed
>    throught those pserialize(9) protected code paths, by calling
>    pserialize_switchpoint() from mi_switch().  This obviously implies
>    that threads can't sleep from within those pserialize(9) protected
>    code paths.  Otherwise that notification has a different meaning.
>
> Yes, that's right.
>
> Ideally, we should avoid blocking much in callouts too.
>
>    I think pserialize_read_{enter,exit}() should explicitly call
>    KPREEMPT_{DISABLE,ENABLE}(), as is done in percpu_{getref,putref}().
>
> Not necessary -- splsoftserial is sufficient to prevent switching, and
> is necessary to block the pserialize cross-call until the reader is
> finished.

I want stronger constraint than softint context.  Softint prevents
threads from being preempted implicitly, but still allows to sleep.  I
want pserialize(9)'ed readers to never sleep (critical section).
KPREEMPT_DISABLE() (and kpreempt_disable()) sets l_noprempt.  Which
seems the official way to declare entering critical section.